1. Field of the Invention
The present invention relates to a semiconductor device.
2. Description of the Related Art
Conventionally, as a semiconductor material, nitride-based compound semiconductor, for example, gallium nitride (GaN)-based compound semiconductor is used for semiconductor elements (hereinafter GaN-based semiconductor elements) for use in high-frequency devices. In the GaN-based semiconductor element, a buffer layer or a doped GaN layer formed by using, for example, Metal Organic Chemical Vapor Deposition (MOCVD) method is provided on a surface of a semiconductor substrate. Recently, based on the recognition of being applicable to power devices for electric power use in addition to high frequency use, a device to which high voltage or large electric current is applied is also under research and development.
High electron mobility transistor (HEMT) is a kind of GaN-based semiconductor element. For example, HEMT has a configuration in which a GaN layer and an aluminum gallium nitride (AlGaN) layer are grown. A two-dimensional electron gas (2 DEG) is generated at an interface of the GaN layer with the AlGaN layer. Carrier density of the 2 DEG can be controlled by adjusting the Al composition ratio and the thickness of the AlGaN layer. In HEMT, the 2 DEG layer becomes an electric-current-flowing path. In addition, in HEMT, a gate electrode having Schottky characteristics with respect to the AlGaN layer is disposed on a part of a surface of the AlGaN layer, and the path made of the 2 DEG layer is switched on and off by controlling gate voltage. A type of HEMT in which the path between the source and the drain becomes conductive when the gate voltage is 0 V is called normally-on type. On the other hand, a type of HEMT in which the path between the source and the drain is not conductive when the gate voltage is 0 V and the path becomes conductive when the gate voltage becomes positive voltage is called a normally-off type. Hereinafter, HEMT etc. made of GaN-based compound semiconductor is called GaN-HEMT etc.
In some cases, an element used in an application circuit is required to be normally-off type from a viewpoint of safe operation of the application circuit. In order to make HEMT normally off type, there is a technique of eliminating a 2 DEG layer partly by injecting ion of fluorine etc. or radiating plasma immediately under a gate electrode, or making the thickness of the AlGaN layer immediately under the gate electrode partly thinner by etching etc. Also, in order to prevent gate leakage, there is a case in which an insulating film is provided under the gate electrode. In normally-off type of HEMT, when the gate is in off state and voltage is applied between the source and the drain, the 2 DEG layer is depleted from a gate end and high withstand voltage can be maintained. Accordingly, since HEMT functions as a high withstand voltage semiconductor element for use with large electric power, research and development for HEMT, as a high frequency and high efficiency semiconductor element for electric power use, are in progress actively in recent years. Also, research and development for high withstand voltage Schottky barrier diode (SBD) using a 2 DEG layer as a conductive layer are in progress in a similar manner.
In order to be used as a semiconductor element for electric power use, high speed operation and low conductive resistance as explained above are great advantages. On the other hand, high reliability that the element will not be broken even various load is applied is required for a semiconductor element for electric power use. There is avalanche tolerance (avalanche withstand voltage) as one of basic characteristics of a semiconductor element. Avalanche tolerance indicates tolerability where an element will not be broken until a predetermined electric current even when voltage equal to or greater than avalanche tolerance is applied to the element and when the element breaks down between the source and the drain due to avalanche breakdown. For example, in J. Kuzmik et al., “Electrostatic discharge effects in AlGaN/GaN high electron mobility transistors”, Applied Physics Letters, Vol. 83, No. 22, 2003, pp. 4655-4657. (hereinafter to be referred to as Document 1), a so-called transmission line pulser (TLP) measurement in which current-voltage characteristics when a voltage stress is applied in short period of time is studied for GaN-HEMT. In this case, the avalanche tolerance of the GaN-HEMT is considered to be low because the GaN-HEMT leads to breakdown without avalanche breakdown after negative resistance occurs rapidly at a certain voltage.
A phenomenon called as collapse is considered to be another problem of GaN-HEMT. Collapse is a phenomenon where, when high voltage is applied to an element, forward resistance increases to a degree, for example, several times as much as an initial value. If collapse occurs, electric power loss in the element increases.